Generate file attachment transactions for explicit Remarkup attachments on common edit pathways

Summary:
Ref T13603. On common edit pathways, extract explicit file attachments from Remarkup. These pathways are affected:

• Objects that use EditEngine and expose a remarkup area via "RemarkupEditField".
• Objects that use EditEngine to generate a comment area.

This is the vast majority of pathways, but not entirely exhaustive.

Test Plan: Created and commented on a task, explicitly attaching images. Saw images attach properly.

Maniphest Tasks: T13603

Differential Revision: https://secure.phabricator.com/D21830